Mermaids are easily one of the most spellbinding fantasy creatures ever. They've held a place in people's imaginations for centuries and there really isn't any sign of that stopping any time soon. They inspire trends all over the place, like those mermaid pillows covered in reversible sequins or mermaid hair. There was a mermaid "documentary" not too long ago that talked about mermaids from a zoological perspective and people were really into it. Maybe that's what inspired this reddit user to ask:
What do you think would be the major differences between a freshwater mermaid and a saltwater mermaid?
Honestly, a lot of the answers were really insightful and well thought out! And a lot were... um ... not that. We grabbed 20 of our favorites for you guys to read.
Saltwater mermaids would show more variance in color and fish shape do to the large habitat (think shark mermaids, clownfish mermaids, lion-fish mermaids, etc), better suited for combat on account of the oceans many predators, and have more worldly knowledge on account of having access to every major coastal city.
Freshwater mermaids would be a bit duller and look similar to other mermaids in the same lake but radically different from mermaids of other lakes. They'd suited for camouflage on account of having to survive in a small space in close to humans. They'd be much more timid that your saltwater mermaid.
I wouldn't think there'd be any difference if mermaids breathe air. They wouldn't have to worry about the whole osmotic balance thing the way a critter with gills would. They'd be more like whales - could go from fresh to brackish to salt water without any real issues.